As part of recent approach to environmental problems, attempts have been made to cause power conversion apparatuses to convert a DC power generated by solar batteries or fuel batteries into an AC power and supply it to domestic loads (to be simply referred to as “loads” hereinafter in this specification) or commercial power systems (to be simply referred to as “systems” hereinafter in this specification) or convert a DC power into a predetermined DC voltage and use it to drive DC loads.
Most power conversion apparatuses used for the above purposes have a function of stepping up the output voltage from a solar battery to a predetermined voltage. The power stepped up is used for a DC load or input to a DC/AC conversion apparatus, converted into an AC power, and then connected to systems.
There is also a method of raising the output voltage from solar batteries by connecting them in series. However, connecting solar batteries in series requires a number of working steps and accordingly increases the cost. In addition, the non-power-generation area of the solar power generation apparatus increases, and the influence of partial shade becomes large.
To solve these problems, a solar power generation apparatus has been developed, which extracts an output power with a high voltage and small current by making the number of series-connected solar batteries as small as possible and highly stepping up the output voltage from the solar batteries.
In this solar power generation apparatus, only a low voltage of about 1 V can be output per solar panel. Hence, a considerably high-step-up DC/DC conversion apparatus is necessary. In addition, the apparatus must incorporate a high-step-up transformer.
Conventionally, a push-pull circuit is used as an example of the circuit scheme of the high-step-up DC/DC conversion apparatus as described above.
To generate a high voltage from a low voltage, for example, an inverter apparatus using a transformer assembly has been proposed. This transformer assembly uses a plurality of transformers whose primary coils are connected in parallel and secondary coils are connected in series to implement high step-up. Arrangements which embody similar transformer assembles have also been proposed.
However, the DC/DC conversion apparatus connected to solar batteries is preferably arranged near the solar batteries to reduce transmission loss of the generated power of the solar batteries. The conversion apparatus is required to be as thin as possible in correspondence with the low-profile shape of the solar batteries.
Especially, a transformer as a constituent component of the DC/DC conversion apparatus greatly affects the apparatus thickness. Although the apparatus thickness is preferably small, it can hardly be reduced in the conventional transformer structure.
FIG. 5 shows a conventional push-pull transformer 501 which has a pin terminal 503 to be used to mount the transformer on a printed circuit board. In this transformer, primary coils 504 and 505 are wound on a winding core and led out toward a pin terminal 503. The end portions are connected to terminals 501a to 501d of the pin terminal 503, thereby forming an output terminal.
When a low-voltage large-current solar battery is used as a power supply, a large current flows to the primary coil of the transformer. Hence, a flat-type copper wire or a thick copper foil is preferably used as the primary coil. However, when such a material is used, it is very difficult to wind the primary coils and lead them out to the pin terminal to form an output terminal, unlike the conventional transformer.
FIGS. 6A and 6B show examples of parallel connection (FIG. 6A) and series connection (FIG. 6B) of conventional transformers on a printed circuit board.
Referring to FIGS. 6A and 6B, reference numerals 601 to 604 denote transformers; 605 to 608, input terminals of push-pull circuits; and 609, a through hole. A portion indicated by a solid line is a land on the upper surface of the printed circuit board, to which the pin terminal or input terminal of a transformer is connected. A portion indicated by a broken line is a land on the lower surface of the printed circuit board.
FIGS. 16 and 17 are schematic views showing connection circuits for the transformers and input terminals shown in FIGS. 6A and 6B.
For the illustrative convenience, switching elements 1601, 1602, 1701, and 1702 connected to terminals 601c, 602d, 604c, and 604d shown in FIGS. 16 and 17 are not illustrated in FIGS. 6A and 6B.
As shown in FIGS. 6A and 6B, when transformers are connected in series or in parallel on a printed circuit board, conductors which connect the input terminals 605 to 608 to the respective terminals cross on the printed circuit board. The conductors must be led to the lower surface of the circuit board through the through hole 609 or the like. This greatly increases the wiring resistance and decreases the efficiency. In addition, the printed circuit board becomes bulky depending on the wiring pattern which connects the terminals of the transformers.
Even when the connection positions of the primary coils and pin terminals in each transformer are changed, the same problems as described above are posed.